The present invention relates to an automatic transmission in which a gear ratio can be set on the basis of a driving condition, for example a load against a power source of a vehicle, a vehicle speed, etc. Especially, this invention relates to a control device of the automatic transmission in which a gear ratio of overdrive gears can be set.
A power source, for example an internal combustion engine or/and an electric motor, etc. are installed in a vehicle. Since characteristics of such a power source do not necessarily satisfy an output power required over the entire range of a vehicle from starting to high speed running, a transmission is mounted in the vehicle in addition to the power source in order to increase or reduce driving torque or a rotation speed of the power source. As one example of the transmission, an automatic transmission, which is automatically controlled to shift gears according to a running condition of the vehicle, is adopted in many cases. Furthermore, in accordance with a required improvement of power output characteristics or a required reduction of fuel consumption of the vehicle, the smallest gear ratio of the automatic transmission tends to be smaller than 1. A revolution of the power source can be reduced in high speed running of the vehicle, owing to the above-mentioned automatic transmission.
In the aforementioned background, an automatic transmission with a plurality of shift gears tends to multiply the number of the shift gears in these days. One example of such an automatic transmission is shown in Japanese Laid-Open Patent Application No. 8-177994. The automatic transmission shown in this Patent Application, has a structure mainly having 3 sets of planetary gears, and 5th gear and 6th gear are respectively overdrive gears. Furthermore, rotating speeds of rotating members of the automatic transmission are restrained.
In the automatic transmission with six forward gears, the gear ratio of the highest gear (i.e. the smallest gear ratio of the automatic transmission) is smaller than a gear ratio of the highest gear ratio of an automatic transmission with five forward gears. Since an engine revolution in a vehicle having the automatic transmission can be reduced, the fuel consumption of the vehicle is improved during high speed driving. Furthermore, since excessive high rotating speeds of rotating members can be avoided, a durability of the automatic transmission is improved.
Since the gear ratio of 6th gear, as the highest gear, is small, however, the driving torque of the vehicle in 6th gear is small. The 6th gear is set when a load of an engine is low (i.e. a throttle angle or accelerator angle is low), and a speed of the vehicle is rather high. Consequently, even though the speed of the vehicle reduces only a few, or the engine load increases only a few by depressing an accelerator pedal of the vehicle, a driving condition of the vehicle enters into the 5th gear range, and a down-shift of the automatic transmission from 6th gear to 5th gear occurs. If the speed of the vehicle is higher or the engine load is lower by pulling back the accelerator pedal after the down-shift happens, the driving condition of the vehicle again enters into the 6th gear range, and the up-shift of the automatic transmission occurs.
As mentioned above, from the viewpoint of fuel efficiency of the vehicle it is advantageous that the automatic transmission has multiplied gears. On the contrary, however, an up-shift or a down-shift of the automatic transmission frequently occurs, caused by only a little fluctuation of a driving condition of the vehicle. Such a phenomenon is called xe2x80x9cbusy-shiftxe2x80x9d. That is, a driver of the vehicle might feel uncomfortable because of the busy-shift.
It is thus one object of the present invention to solve the aforementioned problems. That is, the object of the invention is to provide a control device for an automatic transmission which prevents an automatic transmission from the above-mentioned busy-shift caused by a lower gear ratio of the highest gear, and which enables to effectively and practically use a gear ratio of the highest gear.
A control device for an automatic transmission comprises a selecting device and a highest gear control means. The selecting device selects an automatic shift mode or an independent manual shift mode. The automatic shift mode automatically determines a gear ratio of the automatic transmission on the basis of a driving condition of a vehicle having the automatic transmission. The manual shift mode determines a gear ratio of the automatic transmission by a driver""s intention. The highest gear control means controls the automatic transmission so that the smallest gear ratio set by the manual shift mode is smaller than the smallest gear ratio set by the automatic shift mode.
If the gear ratio of the highest gear of the automatic shift mode is not so small, a shift caused by a change of a driving condition does not easily occur. That is, the busy-shift can be avoided in the automatic transmission. Although the further small gear ratio can not be adopted in the automatic shift mode, it can be set if the manual shift mode is selected. In this case, revolutions of a driving power source is reduced and fuel efficiency can be improved.
A control device for an automatic transmission comprises a shift device and a means for guiding the shift device. The shift device selects a neutral position, a highest gear prohibition position, a highest gear permission position, and a middle speed position. In the neutral position, the automatic transmission does not output a torque to an output shaft of the automatic transmission. In the highest gear prohibition position, the highest gear of the automatic transmission is prohibited. In the highest gear permission position, the highest gear of the automatic transmission is allowable. In the middle speed position, a gear ratio can be set between the largest gear ratio and a predetermined gear ratio larger than the smallest gear ratio of the highest gear prohibition position. The guiding means guides the shift device along a path to the neutral position, a path to the highest gear permission position, and a path to the middle speed position. Each path separately branches off from the highest gear prohibition position and is connected to the neutral position, the highest gear permission position, and the middle speed position.
Since the highest gear is not set in the highest gear prohibition position which is selected by moving a shift device from the neutral position, a slight change of a driving condition in high speed running does not cause a down-shift of the automatic transmission. Consequently, the busy-shift can be avoided. On the contrary, since the highest gear permission position is selected by further moving the shift device, and the highest gear in the highest gear permission position is intentionally set by a manual operation. In other wards, since the highest gear can intentionally be released, the busy-shift can be avoided by the manual operation. Furthermore, even if a gear shift happens at the highest gear running in the highest gear permission position, a driver does not feel uncomfortable, because the driver selects such a position and is ready for the down-shift.
A control device for an automatic transmission comprises a plurality of overdrive gears and a highest gear prohibition means. The highest gear prohibition means prohibits the smallest gear ratio of the overdrive gears of the automatic transmission when a speed of a vehicle having the automatic transmission is equal to or over a predetermined value and a running resistance of the vehicle is equal to or over a predetermined value.
Since the automatic transmission has a plurality of overdrive gears, revolutions of a driving power source can be reduced by selecting the smaller gear ratio among the overdrive gears in high speed driving. The fuel efficiency of the vehicle is then improved. On the other hands, when the vehicle runs on a slope at high speed and a running resistance is high, the smallest gear ratio is prohibited. Since the gear ratio is not so small in this case, a down-shift does not easily occur. Therefore the busy-shift can be avoided.
A control device for an automatic transmission is installed in a vehicle having a driving instruction system. The driving instruction system detects a predetermined scheduled course for driving the vehicle and a road condition of the predetermined scheduled course. The control device comprises a means for controlling a gear shift of the automatic transmission and a highest gear prohibition means. The means controls a gear shift on the basis of the road condition of the predetermined scheduled course. The highest gear prohibition means prohibits the smallest gear ratio of the overdrive gears of the automatic transmission, when the vehicle is driven on the predetermined scheduled course, a speed of the vehicle is equal to or over a predetermined value, and a running resistance of the vehicle is equal to or over a predetermined value.
When a road ahead for high speed running is detected in a predetermined scheduled course and a rather high torque is necessary for the vehicle to run on the road, the highest gear of the automatic transmission is prohibited before the vehicle gets to the road. Since the needed driving torque is secured when the vehicle runs on the road, it can be avoided in advance that the vehicle speed is slower and a shift-down occurs or an up-shift after the shift-down (that is the busy-shift) occurs.
A control device for an automatic transmission is installed in a vehicle which has a following device. The following device follows another vehicle ahead and comprises a shift device and a highest gear prohibition means. The highest gear prohibition means prohibits the smallest gear ratio of the overdrive gears of the automatic transmission when the vehicle is driven by the following device.
Since the highest gear is not set when the vehicle follows another vehicle, enough torque for driving is secured. Consequently, a following and response ability to the vehicle ahead can be improved.